The present invention relates generally to hard-facing coatings, and, more particularly, to an improved method for applying a coating layer to a substrate.
The life and reliability of hydraulic cylinders often depends on having a hardened wear coating on the interior surface of the cylinder bore. This coating provides wear resistance and a bearing capability for the piston head and seal. The piston rod can experience significant side loads during use, often causing a dynamic metal-to-metal contact between the bore wall and the piston head. In addition, abrasive particles accumulate in the piston seal. After millions of inches of piston travel, these entrained particles can dramatically degrade the bore finish, which contributes to increased internal leakage and accelerated wear. By providing a protective hardened coating on the bore wall, the life of the actuator can be dramatically increased.
The common way to provide such a hardened coating is to electrolytically deposit chrome on the cylinder bores. Although chrome provides the desired mechanical wear properties, there are a number of problems attendant its use. First, the electroplating process creates hydrogen embrittlement of the substrate, which reduces the fatigue strength of the faced material. Second, the added wall thicknesses required to reduce stress levels to the point of affording the required life, can add significant weight to the part. Third, chrome processing is recognized to provide an environmental hazard. In the United States, the Environmental Protection Agency is tightening controls on wastestream treatment at plating houses. This has resulted in increased plating costs. The government has also initiated activities aimed at developing alternative coating materials. Fourth, the use of chrome plating represents a significant recurring cost for the actuator manufacturer. Because the coating is not applied uniformly, a build-up of chrome can provide an excessive thickness, particularly on edges and corners, that must be removed. Considerable expense is incurred in grinding down these excessive build-ups. Still, another factor is that ongoing efforts are directed toward characterizing the fatigue damage due to the electroplating process, and to determine acceptable design stress levels.
Accordingly, it would be generally desirable to provide an improved method of providing a hard-facing coating on a substrate, such as the bore wall of a cylinder, in a manner that would overcome these problems.
With reference to the corresponding parts, portions or surfaces of the disclosed embodiments, merely for purposes of illustration, and not by way of limitation, the present invention provides an improved method of applying a particulate material to a substrate. The improved method broadly comprises the steps of removing impurities from a surface of the substrate; forming a coating composition having a bonding material and at least one particulate material; applying the coating composition to the substrate surface; and creating a diffusion bond between the substrate, bonding material and particulate material for generating a continuous interface between the substrate surface and particulate material such that the change in mechanical properties between the substrate and particulate material occurs in a direction normal to the plane of the substrate surface, thereby minimizing residual strain and coefficient of thermal expansion mismatches between the substrate and particulate material, the surfaces of individual particles of the particulate material being chemically wetted by the bonding material; whereby the particulate material and the bonding material comprise a layer on the substrate surface.
The impurities may be removed from the substrate surface by thermal decomposition, chemical decomposition, electrolytic decomposition, oblation by ions, particularly by high-energy beams, ultrasonically, by fluxing, or by some other unspecified means or method.
The coating composition may be applied to the substrate surface by spraying, dipping, painting, tape-casting, or some other unspecified technique.
The coating composition may include an organic binder, and the particulate material may include a material selected from the group consisting of tungsten carbide (WC), titanium carbide (TIC), vanadium carbide (VC), titanium diboride (TiB2), hafnium carbide (HfC), molybdenum carbide (Mo2C, MoC, M3C2) chrome boride (CrB2), silicon carbide (SiC), diamond hafnium diboride (HfB2), zirconium carbide (ZrC) and tantalum carbide (TaC).
The mechanical properties between the substrate and the particulate material are preferable in non-abrupt, smooth, continuous transition along a direction normal to the plane of the interface.
The substrate may be Inconel(copyright) (a registered trademark of Inco Alloys International, Inc., 3200 Riverside Rive, Huntington, W. Va. 25720, U.S.A.), 15-5 PH stainless steel, or some other material.
Accordingly, the general object of the invention is to provide an improved method of applying a particulate material to a substrate.
Another object is to provide an improved method of applying a hard-facing material to a substrate.
Another object is to provide improved coating compositions for use in such methods.